Two Types of Nuclear Oestradiol Binding Sites in Human Myometrium and Leiomyoma during the Menstrual Cycle1)

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Abstract
Two types of oestradiol binding sites were identified in human myometrium and myoma by saturation analysis. Nuclear type I receptors bind Oe with high affinity (KD = 1.4 nM) and low capacity (0.1–2.0 pmole/mg DNA) in a competitive fashion (Hill coeff. = 1.0), while nuclear type II sites bind the hormone with reduced affinity (Kp : ˜20 nM in myometrium and KD : ˜40 nM in myoma), but high capacity (1 – 15 pmole/mg DNA) by positive cooperation (Hill coeff.: 4–5 in myometrium and 2–3 in myoma). Binding properties of type II sites in myoma are similar to those found in endometrium in previous studies of our laboratory, rather than those found in normal myometrium. The concentration of both nuclear Oe binding sites varied with the menstrual cycle. In myometrium, maximal concentration of nuclear type I and type II sites occurred between 10–14 days of cycle, when blood Oe level is the highest. In myoma the concentration of type I receptors was highest during the late follicular phase, but type II sites were uniformly high during the first 14 days of the cycle, In the luteal phase, receptor concentrations were low and apparently unaltered. It is possible that these changes in Oe binding capability of leiomyoma play a role in the pathomechanism.